This invention relates to an optical recording medium using light in reproduction of a recorded data signal, and a recording and/or reproducing apparatus adapted to record a data signal onto the optical recording medium and to reproduce the data signal recorded on the optical recording medium, and more particularly to an optical recording medium capable of reproducing a data signal recorded thereon with a d.c. offset component produced at the time of reproduction of such recorded data signal being removed, and a recording and/or reproducing apparatus using such optical recording medium.
In an optical disc recording/reproducing apparatus using an optical disc as a recording medium, for the purpose of allowing a focused beam spot irradiated onto the optical disc to be positioned on a predetermined recording track at all times, tracking servo is carried out. This tracking servo is a servo to detect a quantity of deviation from a predetermined recording track formed on the optical disc as a tracking error signal to move a beam spot focused on the optical disc within the range of a predetermined deviation. In more practical sense, movement control of a focused beam spot position of laser beam is carried out so that the tracking error signal becomes equal to zero.
As the method of detecting a tracking error signal for the above-described tracking servo, there are, e.g., a push-pull method, a three-beam method, and the like.
The above-mentioned push-pull method is a system of detecting a return light reflected from the substrate of the optical disc by using a bisected photo detector to determine a tracking error signal by using a difference between the detected light quantities.
Meanwhile, in the push-pull method, when the object lens (objective) undergoes displacement in a direction perpendicular to the optical axis of this object lens in accordance with the tracking control, so the center of the bisected photo detector and the center of a diffracted light are not in correspondence with each other, a d.c. component may appear on a tracking error signal. This d.c. component is called a d.c. offset.
This d.c. offset occurs resulting from, in addition to the above-described deviation in the optical axis of the object lens, inclination in a radial direction of the optical disc, unevenness of the shape of grooves forming recording tracks provided in the optical disc, a difference between reflection factor of the groove and that of a mirror portion in which no groove is formed, a difference between a d.c. offset produced in the address area and that produced in the data area of the optical disc, and the like.
A circuit for detecting and correcting this d.c. offset component has been proposed.
In this d.c. offset correction circuit, signals obtained by detecting a return light reflected from the disc surface by using a bisected photo detector are delivered to a differential amplifier. The differential amplifier delivers a difference signal between signals detected by two detecting sections to first to third sample-hold circuits. These sample-hold circuits carry out sampling at timings delivered from a timing generator to the respective circuits.
In order to avoid the influence occurring as the result of the fact that a tracking error signal is lost immediately before the mirror portion formed on the optical disc, the first sample-hold circuit samples and holds a signal level of tracking error signal immediately before the mirror portion. This first sample-hold circuit releases sample-hold state after a light beam is passed through the mirror portion.
Further, the second sample-hold the circuit samples and holds tracking error signal immediately before the mirror portion to output a signal comprised of a tracking error signal immediately before the mirror portion including no d.c. offset component and a d.c. offset component. In addition, the third sample-hold circuit detects only the d.c. offset component included in the tracking error signal by sampling at the mirror portion.
The second and third sample-hold circuits, respectively, deliver output signals to the non-inverting terminal and the inverting terminal of a second differential amplifier. Thus, as a difference between these output signals, the second differential amplifier outputs, to a coefficient multiplying section, the tracking error signal immediately before the mirror portion including no d.c. offset component. The coefficient multiplying section multiplies the output difference, which is delivered from the second differential amplifier, by coefficient K to output it to the other terminal side of a summing amplifier. The summing amplifier is supplied on one end side thereof with an output from the first sample-hold circuit. The summing amplifier adds an output (tracking error signal immediately before the mirror portion) of the first sample-hold circuit and a corrected output signal through the coefficient multiplying section from the differential amplifier to output a corrected tracking error signal through the output terminal.
In order to avoid such a d.c. offset, the optical disc recording/reproducing apparatus employs an offset canceling system based on a mirror portion correcting system in which in detecting a d.c. offset when a tracking servo is carried out to move bisected photo detector and an object lens in one body by using a bi-axial actuator or a uni-axial galvano mirror, etc. so that a focused beam spot falls within a predetermined range, a d.c. offset component is detected at the mirror portion provided on the optical disc to correct it.
However, since this system employs a subtraction system of subtracting a position movement component from a push-pull signal which is a difference output of the bisected photo detector to carry out tracking control, in the case where a coefficient K of the coefficient multiplying section constituting d.c. offset correcting circuit is not suitable, any error would occur in the tracking error signal. Further, since such a coefficient is dependent upon the position on the optical disc, there was the case where any adjustment is required resulting from the difference between the coefficient on the inner circumferential side and that on the outer circumferential side, etc. In such a case, with the circuit configuration of this system, the effect of the d.c. offset correction of the tracking error signal cannot be exhibited, so a circuit configuration and a mechanism for still more making improvement becomes complicated.
An optical disc recording/reproducing apparatus using the three-beam method can provide stable tracking operation, but adjustment for allowing two sub-beams on the both sides of a main beam to be placed at symmetric position relative to the recording track center in inner and outer circumferences of the optical disc is very complex.
Further, the applicant of this application has proposed a Differential Push Pull (hereinafter referred to as DPP) method as the push-pull offset canceling system by spatial division in the publication of the Japanese Patent Application Laid Open No. 94246/1986 (Tokkaisho 61-94246).
In accordance with this method, a pair of light beams are irradiated to an optical recording medium through an object lens with a spacing which is a multiple of odd of substantially one half of the track pitch to allow a pair of emitting beams from the optical recording medium to be respectively incident to a pair of bisected photo detecting elements to obtain a tracking error signal from a difference between respective detected outputs from the pair of bisected photo detecting elements to thereby remove a d.c. fluctuation of a tracking error signal based on movement in a direction perpendicular to the optical axis of the object lens or radial skew of the optical recording medium.
However, in the case of carrying out push-pull offset canceling by the DPP method, since a focused beam is divided into three light beam components by using a diffraction grating similarly to the three-beam method to use that sub-beam for detection of push-pull signal, the utilization efficiency of an emitted laser output becomes poor. For this reason, when a great quantity is required for sub-beam even if the focused beam is divided into three beam components, laser diode LD must emit a laser beam at a higher laser output, e.g., an output increased by 15% according to the above requirement.
Meanwhile, in the case where the reliability of laser diode LD is generally taken into consideration, there is the relationship that power of 2 of a laser output is proportional to inverse number of the life time of the laser diode with the laser output and the life time of the laser diode being as a parameter. Accordingly, hen an attempt is made to improve the laser output, the life time of the laser diode becomes short by the above-described proportional relationship. Further, this method requires a diffraction grating which is an expensive part for dividing a light beam into three light beam components.